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Azeotropic distillation Any of several processes by which liquid mixtures containing azeotropes may be separated into their pure components with the aid of an additional substance (called the entrainer, the solvent, or the mass separating agent) to facilitate the distillation. Distillation is a separation technique that exploits the fact that when a liquid is partially vaporized the compositions of the …

Avogadro’s law The principle that equal volumes of all gases and vapors, under the same conditions of temperature and pressure, contain identical number of molecules; also known as Avogadro’s hypothesis. From Avogadro’s law the converse follows that equal numbers of molecules of any gases under identical conditions occupy equal volumes. Therefore, under identical physical conditions the gram-molecular weights of all …

Autoxidation The slow, flameless combustion of materials by reaction with oxy­gen; it is sometimes spelled autooxidation. Autoxidation is important because it is a useful reaction for converting compounds to oxygenated derivatives, and also because it occurs in situations where it is not desired (as in the destructive cracking of the rubber in automobile tires). Although virtually all types of organic …

Atropine An alkaloid, C17H23NO3, with the chemical structure below. The sys­tematic chemical name is endo-(±)-a-(hydroxymethyl) phenylacetic acid 8-methyl- 8-azabicyclo[3.2.l]oct-3-yl ester, and in phamacy it is sometimes known as dl­hyoscyamine. It occurs in minute amounts in the leaves of Atropa belladonna, A. betica, Datura stramonium, D. innoxia, and D. sanguinea, as well as many related plants. It is chiefly manufactured by …

Atomic structure and spectra The idea that matter is subdivided into discrete and further indivisible building blocks called atoms dates back to the Greek philosopher Democritus, whose teachings of the 5th century B.C. are commonly ac­cepted as the earliest authenticated ones concerning what has come to be called atom­ism by students of Greek philosophy. The weaving of the philosophical thread …

Atomic spectrometry A branch of chemical analysis that seeks to deter­mine the composition of a sample in terms of which chemical elements are present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent atoms which are then probed spectroscopically. In routine atomic spectrometry, a device called the atom source or …

Atomic number The number of elementary positive charges (protons) con­tained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom. The concept of atomic number emerged from the work of G. Moseley, done in 1913-1914. He measured the wavelengths of the most energetic rays …

Atomic nucleus The central region of an atom. Atoms are composed of nega­tively charged electrons, positively charged protons, and electrically neutral neutrons. The protons and neutrons (collectively known as nucleons) are located in a small central region known as the nucleus. The electrons move in orbits which are large in comparison with the dimensions of the nucleus itself. Protons and …

Atomic mass unit An arbitrarily defined unit in terms of which the masses of individual atoms are expressed. One atomic mass unit is defined as exactly 1fi2 of the mass of an atom of the nuclide 12C, the predominant isotope of carbon. The unit, also known as the dalton, is often abbreviated amu, and is designated by the symbol u. …

Atomic mass : The mass of an atom or molecule on a scale where the mass of a carbon-12 (12C) atom is exactly 12.0. The mass of any atom is approximately equal to the total number of its protons and neutrons multiplied by the atomic mass unit, u = 1.6605397 x 10-24 gram. (Electrons are much lighter, about 0.0005486 u.) …